1. Field of the Invention
The present invention relates to the field of photography and, more specifically, to automatic exposure control systems of the type including a follow focus mechanism for selecting scene range dependent exposure aperture values for the flash illumination mode of operation and an aperture trim control mechanism for making fine adjustments to the follow focus selected aperture value.
2. Description of the Prior Art
The present invention is directed to improvements in an automatic exposure control system of the general type disclosed and claimed in U.S. Pat. No. 3,950,766 issued to Irving Erlichman et al on Apr. 13, 1976 and being commonly assigned with the present application.
The above-noted exposure control system operates automatically in both the ambient light and flash illumination modes of operation. Because of its simplicity of construction, relatively small physical size and a design which facilitates assembly by automatic machines, it may be produced economically and it is compatible for use in compact automated cameras.
The exposure control system is characterized by a thin two bladed shutter-aperture mechanism in which the blades are reciprocally driven simultaneously in opposite directions, between first and second terminal positions, for respectively blocking and unblocking the camera's optical path between the variable focus objective lens and the film at the focal plane. As the blades are driven toward the unblocking position, complementary apertures in the two blades are moved relative to one another and are progressively brought into registration to define a progressively changing exposure aperture over the optical path through which image bearing light from the scene is transmitted to a film unit at the focal plane.
In a preferred embodiment the two blades are coupled directly to a reversible stepper motor which is energized with a first train of pulse signals to drive the motor in a direction to open the shutter-aperture blades and is thereafter energized with a second train of pulse signals to drive the blades to the closed or blocking position.
In the ambient light mode of operation, the exposure control system operates under the control of a scene light sensing and integrating circuit. That is, in response to actuating a cycle start switch the stepper motor is energized and the blades are driven stepwise toward the unblocking position causing the exposure aperture to progressively increase in size with each step. While the blades are being so driven, the light sensing circuit integrates the sensed scene light intensity and at a predetermined voltage level provides a trigger signal which causes the pulse signals to the stepper motor to be reversed in pole exciting sequence thereby reversing the stepper motor which drives the blades back to the blocking position to terminate the exposure interval.
In the flash illumination mode the system functions in a somewhat different manner. Because the duration and the light intensity of the flash source are known parameters, the system is designed to operate in a follow focus mode. That is, exposure aperture value is preselected in proportion to scene range or distance from the camera and the aperture is set automatically in response to focusing the objective lens.
The disclosed follow-focus system includes a blade position sensor or electrical switch component that is mounted for movement along a sensor path that is substantially parallel to the path of travel of at least one of the two shutter aperture blades. Coupled to the camera's lens focusing wheel in cam follower relation, a follow-focus linkage moves the switch component to a predetermined location on the sensor path that corresponds to a blade position wherein the selected exposure aperture is in accordance with the scene range.
In operation, the start switch is actuated and the stepper motor is energized with the first train of pulse signals to drive the blades from the blocking position toward the unblocking position. At the preselected aperture value, the one blade engages and actuates the switch member which forms part of a control system. Upon switch actuation the stepper motor is automatically stopped or deenergized thereby holding the blades in the position defining the range selected aperture. The flash source is fired to expose the film unit and in response to the firing a signal is provided which causes the stepper motor to be energized with the second pulse train signal to drive the blades to the closed position.
Although the automatic exposure control disclosed in the previously noted U.S. Pat. No. 3,950,766 functions in a satisfactorily manner it does have a limitation in that the programmed exposure response of the system cannot be varied or trimmed by the operator to more closely match the exposure response to the sensitometric characteristics of the film or to exert personal control over the exposure by adjusting for intentional underexposure or overexposure to add a creative touch to the photograph or to make the proper adjustments for scene lighting conditions that depart significantly from the "average" conditions for which the exposure control system is preprogrammed.
Trim controls for making fine adjustments in the exposure response of such automatic control systems are, of course, well known in the prior art.
In general, exposure trim for the ambient light operation is provided by adjusting the apparent response of the light sensing and integrating circuit to change exposure interval. The most common way of doing this is to pivotally mount a variable neutral density filter wedge in front of the light circuit's photocell which the operator sets manually before initiating the film exposure operation. The follow focus systems for flash mode exposure generally operate in the exposure aperture selection mode whereby the blades are momentarily stopped at the selected maximum scene range related exposure aperture at or just before the flash fire phase. Commonly flash mode trim is accomplished by a trim control that is effective to vary the position (and thereby the exposure aperture size) at which the blades are stopped in preparation for flash fire.
U.S. Pat. Nos. 3,952,312 and 4,007,468, both commonly assigned with the present application, are representative of automatic exposure systems with trim controls for use with twin reciprocally driven shutter-aperture blades. In the ambient light mode trim is provided optically by means of an adjustable neutral density filter wedge in front of the photocell. For flash operation the follow focus system includes a blade stop pin that is moved into the path of travel of at least one of the blades at a position related to focus distance. Trim is provided by a manually operable linkage that moves the pin position independently of the pin positioning controlled by the follow focus device for optionally varying or trimming the follow focus selected aperture value. While both of the systems function satisfactorily, the follow focus device and trim mechanism are somewhat complex mechanically and tend to be relatively expensive to manufacture. For example, in the previously noted U.S. Pat. No. 3,952,312 the follow focus system includes a separate solenoid and stop pin linkage for moving the exposure aperture setting stop pin into and out of the blade's path of travel for flash illumination and ambient light modes respectively. In the previously noted U.S. Pat. No. 4,007,468 the follow focus system employs a spring biased mechanical linkage coupled between a blade stop pin and the flash socket such that the linkage moves the pin into the blade path when a flash source is inserted into the socket and moves it out of the blade path when the flash source is removed therefrom.
U.S. Pat. No. 3,433,144, commonly assigned with the present application, discloses an automatic exposure control system having an exposure aperture selecting follow focus system wherein an electrical switch means for shutter operation and flash fire is movable along a first sensor path of travel in response to lens focusing for aperture selection. Aperture trim is provided by a trim control which operates independently of the follow focus device and is effective to move the switch means along a second path disposed in substantially transverse intersecting relation to the first sensor path. The effective maximum exposure aperture is set or determined by the position of the switch means along both the first and second path. That is the location of the switch means can be described in a two coordinate system which determines the size of the maximum exposure aperture selected by the two independent inputs of the follow focus system and the trim control.
One inherent disadvantage of the exposure control system disclosed in U.S. Pat. No. 3,433,144 is that the two coordinate positioning of the shutter actuating and flash fire switch means is remote in location from the aperture setting blades and the switch means position is an indirect measure of the actual exposure aperture defined by the blades. This device employs a separate shutter and aperture defining blades. The pair of aperture defining blades are coupled to a pivoted shutter actuator link. As the link is progressively displaced from its normal starting position toward a terminal position, the link drives the two blades relative to each other to define an exposure aperture that progressively increases in size. One terminal or contact of the switch means is mounted on the actuating link. The other contact is mounted for movement relative to the actuator link in response to movement of the lens focusing device and trim control.
When the actuator link is depressed, it travels about its pivot point and the one switch contact thereon engages the other switch contact to close a switch which electrically trips the separate shutter blades and fires the flash.
The engagement of the switch means is an accurate measure of the displacement of the actuator link. But, it is only an indirect measure of exposure aperture size because of the coupling between the actuator link and the exposure aperture defining blade. Thus in order to prevent significant deviations of aperture size from camera to camera all of the parts making up the actuator link, aperture blades, switch means and the mounting for the same must be manufactured to very close tolerances and assembled very carefully thereby making such a system relatively costly to produce.
Therefore, it is an object of the present invention to provide an automatic exposure control system for use in a photographic camera and having a follow focus mechanism and exposure aperture trim control that is simple in construction and relatively easy to manufacture, assemble and test.
It is another object of the invention to provide such a system which includes a blade mechanism including at least one blade member mounted for movement between positions blocking and unblocking the camera's optical path and in which the blade defines progressively changing aperture values and the follow focus and trim control provide a direct measure of the blade position to establish a preselected aperture value.
It is yet another object of the invention to provide such an exposure control system wherein the follow focus and trim controls are actuable to move a blade sensor independently along mutually intersecting transverse paths and the blade member has an indicator thereon that cooperates directly with the sensor to accurately indicate blade position associated with a preselected aperture value.